The invention relates to moldable heat-curable compositions, based especially on unsaturated polyester resin, for obtaining molded articles with a practically zero final shrinkage (generally called "low profile"-shrinkage of approximately between 0 and 0.05%) or a slight expansion, that is to say an increase in size, for example, from +0.2% to +0.09%.
Conventional unsaturated polyester resins usually include an unsaturated polyester diluted in a monomer containing ethylenic (vinyl or acrylic) unsaturation in order to adjust the viscosity as a function of the intended application. This monomer serves as a solvent for the polyester and for the crosslinking agent. A shrinkage in volume is observed during the crosslinking (or curing) of the polyester and of the monomer. This shrinkage results in cracks, faults in appearance and distortion of the articles. Shrinkage-compensating agents are therefore added, which are generally thermoplastic substances. If introduced in the form of powder or diluted in styrene, these agents are of a polystyrene, polyethylene, polyvinyl chloride or polymethyl methacrylate nature, resulting in linear shrinkages, after molding, of 0.2% to 0.1% (low shrink) or else of polyvinyl acetate, saturated polyester or elastomer kind, resulting in shrinkages close to zero (low profile). Whether these shrinkage-compensating additives are or are not soluble in the resins before crosslinking, they produce a phase separation at the time of the crosslinking. The microdomains formed give rise to a vacuum volume compensating the shrinkage of the polyester.
Heat-curable compositions which have zero shrinkage and are prepared from shrinkage-compensating additives known hitherto are impossible to pigment homogeneously; in particular, whitish marbling effects. are observed. It is therefore not possible to obtain molded articles containing pigments or dyes distributed homogeneously in bulk. This presents a problem when it is desired to manufacture colored molded articles, for example in the field of household electrical appliances.
Moldable heat-curable compositions also find an application in fields in which it is desired to obtain molded articles with a perfectly smooth surface which can be coated, for example, with a layer of paint, varnish or with a metallic substance: this is the case, in particular, with motor vehicle components, for example components for the bodywork and reflectors for lighting devices.
Bodywork components for motor vehicles are manufactured by molding, for example compression or injection molding, of heat-curable materials of the BMC (Bulk Molding Compound) type or of the SMC (Sheet Molding Compound) type including chiefly an unsaturated polyester resin, inorganic fillers and reinforcing fibers and at least one shrinkage-compensating agent. This agent is particularly important in the case of the BMCs or the SMCs intended to form bodywork components, because in some cases it is necessary to obtain zero or practically zero shrinkage (low profile) and, in other cases, a slight expansion in order to meet the requirements of class A of the motor vehicle sector (for example obtaining a perfectly smooth surface without shrinkage cavities).
Molding components intended for motor vehicles, for example those intended for the bodywork or components of the engine compartment must be capable of being painted. Painting of components made of polyester generally requires the deposition of a bonding coat (also called a primer coat) which promotes the adhesion of the coats of paint and improves the surface quality before they are applied. In the case of paints applied, for example, by electrostatic gun-spraying a conductive coat must be deposited beforehand.
To avoid this pretreatment, thought has been given to introducing conductive pigments into the heat-curable mixture. However, as already indicated, with compositions containing shrinkage-compensating agents such as those employed hitherto to obtain shrinkage of the low-profile type or a slight expansion, obtaining a homogeneous pigmentation is difficult and, in the case of a molded article, this results in a surface quality which is not suited for the deposition of a smooth and uniform coat of paint by electrostatic gun-spraying.
Heat-curable materials can also be used for the manufacture of (paraboloid) reflectors for lighting devices, for example for motor vehicles. The reflector must have a reflective surface of very good quality. For this purpose the concave inner surface of the reflector is covered with a thin coat of aluminum. Before the deposition of this coat (aluminum coating), the molded articles are subjected to a preliminary treatment, for example the deposition of a varnish, intended to impart perfect surface quality to the article and to promote the adhesion of the aluminum coating. Another possible treatment is to make the surface antistatic and to perform a deposition of powder at elevated temperature, to obtain a film which promotes the adhesion of the aluminum coating.
These treatments are costly and it would naturally be desirable, for obvious reasons of economy, to make it possible to eliminate the pretreatments of the articles molded from BMC or SMC heat-curable materials, regardless of whether this is the treatment before the deposition of paint or else that before the coating with aluminum, and hence to obtain moulded articles whose surface quality allows the aluminum or paint to be deposited directly.